A technology is known for preventing a liquid refrigerant from stagnating in a compressor. With this technology, during a period of non-operation of a heat pump device used in an air conditioner or the like, the motor winding is energized without driving the compressor motor (hereinafter referred to as “locked energization”) and the compressor is heated to thereby vaporize and discharge the liquid refrigerant. For example, there is a technology for, when a compressor is under operation standby, supplying an alternating-current voltage having a frequency of approximately 20 kilohertz, which is higher than a normal frequency during a normal operation, to a compressor motor to thereby prevent a refrigerant in the compressor from being liquefied by making use of heat generated due to the switching loss of the switching elements from which the inverter is configured and heat generated by the motor (for example, Patent Literature 1).
In the case of an IPM (Interior Permanent Magnet) motor, winding inductance of a rotor changes depending on the position of the rotor. For example, the following technology is disclosed for preventing a refrigerant from stagnating in a compressor. With this technology, for example, when a predetermined time elapses while the temperature of a refrigeration cycle is equal to or lower than a predetermined value, an alternating-current voltage having a frequency of 14 kilohertz or more, which is higher than a normal frequency during a normal operation, is supplied to the motor in the compressor while shifting the phase of the alternating-current voltage to thereby efficiently heat the liquid refrigerant (for example, Patent Literature 2).
Moreover, for example, a technology is disclosed for attempting to keep the amount of heat of the compressor constant irrespective of the effects due to manufacturing variations and environmental variations. With this technology, a section where an electric current flowing to the motor winding is relatively stable near a peak is set as a current detection section, and, on the basis of the peak current value detected at this timing, an optimum voltage command value for obtaining the power necessary for vaporizing and discharging a refrigerant stagnated in the compressor is calculated (for example, Patent Literature 3).
To keep the amount of heat of the compressor constant, i.e., to keep the amount of power supplied to the compressor motor constant and reliably prevent the refrigerant from stagnating in the compressor, it is necessary to more accurately detect the electric current flowing to the motor winding. However, when the frequency with which the locked energization of the compressor motor is performed increases, the section where the electric current flowing to the motor winding is relatively stable near the peak decreases and current detection accuracy deteriorates. Even when an analog current value detected at a sampling cycle undergoes AD-conversion in order to perform current detection, if the frequency with which the locked energization of the compressor motor is performed is high, the number of samples detectable during one cycle decreases. Therefore, to improve the detection accuracy, a microcomputer or the like capable of performing sampling at a high sampling rate is necessary. Therefore, for example, a technology is disclosed for equally dividing the sampling cycle when performing the AD conversion into a plurality of sampling cycles and performing current detection while shifting a plurality of sampling timings one by one to detect power with a high accuracy equivalent to the detection accuracy of sampling performed at a sampling frequency that is doubled or more (for example, Patent Literature 4).